Gradient high-performance thin-layer chromatography for characterizing complex hydrocarbon-containing products

Abstract

Classic saturates, aromatics, resins, and asphaltenes (SARA) analysis has long enabled the chromatographic separation of complex hydrocarbon-containing products into these four families. This simplification often does not provide sufficient or correct information on the relationship between the composition of the products and the process variables studied. The obtention of profiles by gradient automated multiple development (AMD) is an interesting alternative to obtain extended SARA separations with increasing level of complexity for heavy petroleum products that adequately represent the entirety of the sample. An optimized 20-step gradient based on tetrahydrofuran (THF)‒dichloromethane‒n-heptane over a total migration distance of 83 mm is proposed here to characterize all types of heavy oil products. The combination of ultraviolet (UV)–densitometry (at 201, 228, and 273 nm), fluorescence detection by intensity changes using berberine cation, together with on-plate recording of UV spectra of separated peaks and the comparison with those of standards, allow for a deep characterization of chemical families. In the apolar zone of the chromatogram saturates, naphthenes, naphteno-aromatics, alkyl-aromatics, and aromatics with different degrees of condensation are detected. The use of THF, both in the plate precleaning stage and in the first stages of the gradient, is responsible for the satisfactory resolution of the resins in the polar zone. AMD chromatographic zones can be related to peaks from hydrocarbon groups in classical SARA. The application of this gradient to different types of products allowed for the relation of the profiles of bitumen to their origin, differentiation of the resin part trapped in asphaltenes, control performance of deasphalting in hydrorefining products, and the control of the composition of base oils.